This invention relates to coated pigmented phosphors. More particularly, it relates to silicate coated inorganic pigmented phosphors and to the process for producing the phosphors.
Pigmented phosphors are well known in the art of color cathode ray tube manufacture. In U.S. Pat. Nos. 4,020,231 issued Apr. 26, 1977 and 4,128,674 issused Dec. 5, 1978, both patents assigned to the same assignee as the present invention, use is made of coprecipitation of silica and pigment. In U.S. Pat. No. 4,473,634, also assigned to the same assignee as the present invention, use is made of an aqueous insoluble silicatebased glass matrix.
In U.S. Pat. Nos. 4,020,231 and 4,128,674 in which a positive sol colloidal silica is used, the degree of bonding between silica, pigment, and phosphor is weak and pigment can be lost during the slurry screening or dusting operation in making a television screen.
In U.S. Pat. No. 4,473,634, phosphor, pigment, and a silicate binder such as potassium silicate or sodium silicate solution are mixed in a steam kettle and evaporated to dryness. The dried powder is then baked for two hours at about 450.degree. C. The resulting pigmented phosphor is water washed, filtered, and dried. This results in a phosphor with the pigment particles strongly bonded to the phosphor particles. However, in this process it is difficult to achieve uniformly dispersed pigment covering the surface of the phosphor particles. Lumps or clumps of pigment can occur and this is objectionable in the phosphor screening operation.